CSC 4900 Computer Networks:

Transcription

1 CSC 4900 Computer Networks: Professor Henry Carter Fall 2017 Villanova University Department of Computing Sciences

2 Review Last week we talked about design principles, and the application protocols HTTP and DNS Text commands sent over a port (recall telnet example) Difference in statefullness HTTP is primarily a pull protocol Villanova University Department of Computing Sciences 2

3 Web and HTTP First, some vocabulary: Web page consists of objects Object can be HTML file, JPEG image, Java applet, audio file, Web page consists of base HTML-file which includes several referenced objects Each object is addressable by a URL Example URL: host name path name Villanova University Department of Computing Sciences 3

4 HTTP overview HTTP: hypertext transfer protocol Web s application layer protocol client/server model client: browser that requests, receives, displays Web objects server: Web server sends objects in response to requests Villanova University Department of Computing Sciences 4

5 HTTP connections Nonpersistent HTTP At most one object is sent over a TCP connection. HTTP/1.0 uses nonpersistent HTTP Persistent HTTP Multiple objects can be sent over single TCP connection between client and server. HTTP/1.1 uses persistent connections in default mode Villanova University Department of Computing Sciences 5

6 Nonpersistent HTTP Suppose user enters URL (contains text, references to 10 jpeg images) time 1a. HTTP client initiates TCP connection to HTTP server (process) at on port HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object somedepartment/home.index 1b. HTTP server at host waiting for TCP connection at port 80. accepts connection, notifying client 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket Villanova University Department of Computing Sciences 6

7 Nonpersistent HTTP (cont.) 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 4. HTTP server closes TCP connection. time 6. Steps 1-5 repeated for each of 10 jpeg objects Villanova University Department of Computing Sciences 7

8 Non-Persistent HTTP: Response time Definition of RTT: time to send a small packet to travel from client to server and back. Response time: one RTT to initiate TCP connection one RTT for HTTP request and first few bytes of HTTP response to return file transmission time total = 2RTT+transmit time Villanova University Department of Computing Sciences 8

9 Persistent HTTP Nonpersistent HTTP issues: requires 2 RTTs per object OS overhead for each TCP connection browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP server leaves connection open after sending response subsequent HTTP messages between same client/server sent over open connection Persistent without pipelining: client issues new request only when previous response has been received one RTT for each referenced object Persistent with pipelining: default in HTTP/1.1 client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects Villanova University Department of Computing Sciences 9

10 Method types HTTP/1.0 GET POST HEAD asks server to leave requested object out of response HTTP/1.1 GET, POST, HEAD PUT uploads file in entity body to path specified in URL field DELETE deletes file specified in the URL field Villanova University Department of Computing Sciences 10

11 User-server state: cookies Many major Web sites use cookies Four components: 1) cookie header line of HTTP response message 2) cookie header line in HTTP request message Example: Susan access Internet always from same PC She visits a specific e-commerce site for first time When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID 3) cookie file kept on user s host, managed by user s browser 4) back-end database at Web site Villanova University Department of Computing Sciences 11

12 Cookies: keeping state (cont.) Villanova University Department of Computing Sciences 12

13 Cookies (continued) What cookies can bring: authorization shopping carts recommendations user session state (Web e- mail) Cookies and privacy: aside cookies permit sites to learn a lot about you you may supply name and e- mail to sites How to keep state : Protocol endpoints: maintain state at sender/receiver over multiple transactions cookies: http messages carry state Villanova University Department of Computing Sciences 13

14 Web caches (proxy server) Goal: satisfy client request without involving origin server user sets browser: Web accesses via cache browser sends all HTTP requests to cache object in cache: cache returns object else cache requests object from origin server, then returns object to client Villanova University Department of Computing Sciences 14

15 More about Web caching Cache acts as both client and server Typically cache is installed by ISP (university, company, residential ISP) Why Web caching? Reduce response time for client request. Reduce traffic on an institution s access link. Internet dense with caches: enables poor content providers to effectively deliver content (but so does P2P file sharing) Villanova University Department of Computing Sciences 15

16 Caching example Assumptions average object size = 100 Kbits avg. request rate from institution s browsers to origin servers = 15/sec avg data rate to browsers: 1.5 Mbps access link rate: 1.54 Mbps RTT from institutional router to any origin server: 2 sec Consequences utilization (traffic intensity) on LAN = % utilization on access link = ~97% total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds Villanova University Department of Computing Sciences 16

17 Caching example (cont) Possible solution increase bandwidth of access link to, say, 154 Mbps Consequences utilization on LAN = % utilization on access link = ~1% Total delay = Internet delay + access delay + LAN delay = 2 sec + msecs + msecs = 2.02 sec So... how much is this going to cost us? Villanova University Department of Computing Sciences 17

18 Caching example (cont) Install cache suppose cache hit rate is % requests satisfied at cache, 60% requests satisfied at origin Consequence access link utilization: 60% of requests use access link data rate to browsers over access link = (0.6*15) * 100 Kbps =.9 Mbps utilization = 0.9/1.54 =.58 average delay = 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache) = 0.6 ( ) (0.01) = ~ 1.3 secs less than with 154 Mbps link (and cheaper too!) Villanova University Department of Computing Sciences 18

19 Conditional GET Goal: don t send object if cache has up-to-date cached version cache: specify date of cached copy in HTTP request If-modified-since: <date> server: response contains no object if cached copy is up-todate: HTTP/ Not Modified Villanova University Department of Computing Sciences 19

20 Chapter 2: Application layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 Electronic Mail 2.4 DNS 2.5 P2P Applications Villanova University Department of Computing Sciences 20

21 Electronic Mail Three major components: user agents mail servers simple mail transfer protocol: SMTP User Agent a.k.a. mail reader composing, editing, reading mail messages e.g., Outlook, Thunderbird, iphone mail client outgoing, incoming messages stored on server Villanova University Department of Computing Sciences 21

22 Electronic Mail: mail servers Mail Servers mailbox contains incoming messages for user message queue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send messages client: sending mail server server : receiving mail server Villanova University Department of Computing Sciences 22

23 Electronic Mail: SMTP [RFC 2821] uses TCP to reliably transfer message from client to server, port 25 direct transfer: sending server to receiving server three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII Villanova University Department of Computing Sciences 23

24 Scenario: Alice sends message to Bob 1) Alice uses UA to compose message and to 2) Alice s UA sends message to her mail server; message placed in message queue 3) Client side of SMTP opens TCP connection with Bob s mail server 4) SMTP client sends Alice s message over the TCP connection 5) Bob s mail server places the message in Bob s mailbox 6) Bob invokes his user agent to read message Villanova University Department of Computing Sciences 24

25 Sample SMTP interaction S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C:. S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection Villanova University Department of Computing Sciences 25

26 Mail message format SMTP: protocol for exchanging msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: different from SMTP commands! body the message, ASCII characters only header body blank line Villanova University Department of Computing Sciences 26

27 Try SMTP interaction for yourself: telnet gmail-smtp-in.l.google.com 25 see 220 reply from server enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands Send messages to Try including the following headers in your data: From, Date, Subject, To Villanova University Department of Computing Sciences 27

28 SMTP: final words SMTP uses persistent connections Just like...? SMTP requires message (header & body) to be in 7-bit ASCII SMTP server uses CRLF.CRLF to determine end of message Comparison with HTTP: HTTP: pull SMTP: push both have ASCII command/response interaction, status codes HTTP: each object encapsulated in its own response msg SMTP: multiple objects sent in multipart msg Villanova University Department of Computing Sciences 28

29 Message format: multimedia extensions MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content type MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data From: To: Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data base64 encoded data Villanova University Department of Computing Sciences 29

30 Mail access protocols SMTP: delivery/storage to receiver s server Mail access protocol: retrieval from server POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server HTTP: Gmail, Hotmail, Yahoo! Mail, etc. Villanova University Department of Computing Sciences 30

31 POP3 protocol authorization phase client commands: user: declare username pass: password server responses +OK -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete quit S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on C: list S: S: S:. C: retr 1 S: <message 1 contents> S:. C: dele 1 C: retr 2 S: <message 1 contents> S:. C: dele 2 C: quit S: +OK POP3 server signing off Villanova University Department of Computing Sciences 31

32 POP3 (more) and IMAP More about POP3 Previous example uses download and delete mode. Bob cannot re-read e- mail if he changes client Download-and-keep : copies of messages on different clients IMAP Keep all messages in one place: the server Allows user to organize messages in folders IMAP keeps user state across sessions: names of folders and mappings between message IDs and folder name POP3 is stateless across sessions Villanova University Department of Computing Sciences 32

33 Next Time... Textbook Chapter 2.5, 2.7 Remember, you need to read it BEFORE you come to class! Homework Homework #1 was due TODAY! Villanova University Department of Computing Sciences 33